1. Field of the Invention
The present invention relates to overrunning ratchet and pawl clutches. More particularly, the present invention relates to a one-piece compliant overrunning ratchet and pawl clutch with a centrifugal throw-out configuration.
2. State of the Art
One-way overrunning clutches come in several forms, and are widely used in a variety of applications. The four main types include the spring clutch, the roller or ball clutch, the sprag clutch, and the ratchet and pawl clutch. Such clutches are commonly used for example in starters for engines, and in other types of machines for conversion of oscillating rotary motion to continuous rotary motion, for backstopping, indexing, or one-way operation.
One type of overrunning clutch of particular interest is the ratchet and pawl clutch, a simplified typical embodiment of which is shown in FIG. 1. As shown in FIG. 1, ratchet and pawl clutches typically comprise a ratchet wheel 2 with engaging teeth 3 disposed about the perimeter of the wheel, and several pawls 4 located around the perimeter of the ratchet wheel 2. The pawls 4 are rotatably connected via pins 5 to a rotatable pawl wheel 6 such that the pawls 4 may be rotated to engage the engaging teeth 3 of the ratchet wheel 2. A spring 7 is attached to each pawl to rotate the free end of the pawl into engagement with the teeth 3 of the ratchet wheel 2 when the pawl wheel rotates in the engaging or torque output direction (counter-clockwise in FIG. 1). When the pawl wheel rotates in the overrun or free-wheeling direction (clockwise in FIG. 1), the pawls 4 are forced out of engagement with the teeth by the cam surface 8 of the engaging teeth, and thus slide along the teeth without engaging.
As can be seen from FIG. 1, in their simplest form ratchet and pawl clutches typically comprise many parts. A typical overrunning ratchet and pawl clutch requires a minimum of 3 parts per pawl, including the pawl itself, the pin which provides the rotational axis of the pawl, and a spring to bias the pawl toward the engaging teeth. In addition, the pawls must be secured to a pawl wheel which is rotatably disposed about the ratchet wheel. Because of this high part count, the manufacturing cost of prior ratchet and pawl clutches has prohibited their use in many applications. The high part count also significantly increases the weight of such clutches. There is thus a need for an overrunning ratchet and pawl clutch with a smaller part count, so as to reduce the weight, complexity, and manufacturing cost of such clutches.
Overrunning ratchet and pawl clutches incorporating a single piece pawl hub or similar construction are known in the art. For example, U.S. Pat. Nos. 5,000,721 to Williams, 4,570,769 to Isaka, and 3,486,597 to Carlton each disclose a one-way ratchet or clutch mechanism comprising two principle parts. However, there are a number of operating concerns which these prior art devices do not address. First, the moving parts experience significant wear due to sliding contact between the ratchet and the pawls in the overrun mode, thus shortening the useful life of the clutch. Because of the constant friction between the parts, such clutches need constant lubrication, experience significant wear, and are also quite noisy when operated in the overrun mode.
It would therefore be desirable to have a simple overrunning ratchet and pawl clutch with a throw-out mechanism to reduce or eliminate contact between the ratchet wheel and the pawls when in the overrun mode, and thus reduce wear and noise. While clutches with throw-out mechanisms are known in the art, a one-piece ratchet and pawl clutch with centrifugal throw-out of the pawls is not known in the prior art. Furthermore, in many prior art clutches with a throw-out feature, the overrun torque, that is, the torque required to cause the pawls to rotate away from contact with the pawl wheel, is relatively large compared to the output torque that can be handled by the clutch. Thus it would be desirable to have an overrunning ratchet and pawl clutch which will have an overrun torque as close to zero as possible, so as to maximize the ratio between the output torque of the clutch and the overrun torque. This will allow the pawls to "throw-out" under centrifugal force when the pawl wheel rotates in the overrun direction at a relatively low speed.
Backlash is also a major concern with these clutches. When the pawl wheel is reversed from the free-wheeling or overrun direction to the torque output direction, the wheel will rotate some distance before the pawls engage the ratchet teeth. This rotation before engagement is termed backlash, and can impose substantial stress on the components of the clutch. Prior ratchet and pawl clutches have a significant amount of backlash, which causes higher impact loads and less predictable motion of the clutch. It would therefore be desirable to have a simple overrunning ratchet and pawl clutch mechanism which is configured to reduce the maximum possible angular rotation of the pawl wheel when switching from the overrun mode to the engagement mode, so as to reduce backlash.
It would also be desirable to have a micromechanical overrunning ratchet and pawl clutch incorporating the features described above, and being of a size on the order of 10's of microns up to 2 millimeters in overall size. Such microscopic parts must be simple enough in design that they can be fabricated by photolithography or similar techniques, and such that no assembly of the finished device is required, but are very useful in the creation of micromechanical devices.